1. Field of the Invention
The present invention relates to a photovoltaic-chargeable secondary battery system and, more particularly, to a photovoltaic-charged secondary battery system, in which an electrode for optical power generation and an electrode for charging and discharging generated electrical energy are integrated into a single cell structure, and the potential difference between the electrodes is systematically controlled, thus maximizing the conversion efficiency of optical energy, maximizing the utilization rate of cell energy, and extending the life span of the battery.
2. Description of the Related Art
Information communications between individuals become very important in the 21st century knowledge-based society. Accordingly, there have been steady efforts to create a ubiquitous society by building ultra-high speed communication networks and knowledge sharing systems, and thus there arises a necessity for developing energy sources suitable for communication systems of the ubiquitous era.
Most of today's energy supply systems are large scale stationary sources in one-way direction such as nuclear power, waterpower and thermal power generation, and have problems such as inefficiency caused by an imbalance between generation and consumption, disposal of surplus electric power, environmental destruction, and large scale power transmission.
To solve such problems, the energy supply systems should be shifted to a new paradigm for the energy production and consumption of the next generation. That is, it is expected that the energy supply systems will be developed from the one-way structure, in which the power generation and transmission are provided from the stationary generation facilities in a large scale, to individual and independent structures, in which the power generation and consumption are provided in a small scale. Accordingly, it is necessary to create a new concept of power system in which the power generation and storage techniques are merged.
Meanwhile, the most leading secondary battery is a lithium secondary battery including cathode made of a metal oxide and an anode made of carbon.
Since the conversion of chemical energy into electrical energy is reversible in the lithium secondary battery, it is possible to repeat charge and discharge; however, there are limitations to an increase in capacity per unit volume and unit weight according to theoretical capacity limits of electrode active materials. Moreover, since a periodic charge using a charger is required, there is much inconvenience in use. In addition, although extensive research aimed at upsizing the secondary battery has continued to progress, there are a lot of problems to be solved such as safety. [Byung-won Jo, “Analysis on Technologies and Market Trends of Next-generation Secondary Battery for Hybrid Electric Vehicle”, 2007' Next-generation Battery Technologies/Market Trends Analysis Seminar, Korea International Battery & Cell Industry Exhibition 2007, May 9-10, 2007, Seoul].
Meanwhile, photovoltaic cells are broadly classified silicon cells and compound semiconductor cells which directly convert light energy into electrical energy through the photovoltaic effect using a p-n junction in a semiconductor having the same structure as a diode [Su-hong Lee, “Polymer Science and Technology, 17(4), 400-406(2006)]. With extensive research on the photovoltaic cells having continued to progress for the past decades, various types of electrodes as well as materials have been developed, and there are significant improvements in photoelectric conversion efficiency. However, there are limitations on the field of energy utilization due to low energy density, low conversion efficiency in the unit cell, and low output voltage.
Accordingly, in the initial stage, the research aimed at associating the photovoltaic cell capable of generating electrical power using light with a storage battery storing the electrical energy was carried out in preparation for the case where the photovoltaic cell could not generate electrical power in night or on rainy days. Such a photovoltaic cell associated with the storage battery is applied to a streetlight, an emergency telephone, and the like, which can be used without an external power supply [In-sun Seo, Change-geol Park, Kang-hee Kim, Young-il Kwon, Young-seo Park, National Strategic Industry Analysis Report “Photovoltaic Cell”, Korea Institute of Science and Technology Information, BW125, December 2000].
Thus, extensive research aimed at associating the photovoltaic cell with the storage battery has been carried out. For example, Japanese Publication Nos. JP2005-079031, JP2004-241228 and JP2005-209458 by Miyasaka et al. disclose a photoelectric cell in which an electrode performance for optical power generation and an electrode performance for charging are integrated into one electrode material. In such a photoelectric cell, a photoelectrode layer and a counter layer are laminated and integrated at both sides of an ionic electrolyte layer as an intermediate layer, in which the photoelectrode layer is structured by a combination of a photosensitive semiconductor and a solid electrode active material making an oxidation-reduction reaction in electrically coupling with the semiconductor, thus providing a secondary battery capable of optical charging and an electrochemical capacitor. It is disclosed that, in the event that TiO2 is used as a photosensitive material incapable of absorbing visible light, the charging capacity is as low as 0.1 to 0.8 mAh/g; however, in the event that the compound semiconductor (CdS) capable of absorbing visible light with a wavelength of 400 nm or more, the charging capacity is increased to 2.1 mAh/g.
Moreover, Japanese Patent Publication Nos. JP2001-023701, JP2001-10294, JP2001-043903 and JP2001-076768, and Korean Patent Publication No. KP2001-0015248 by Watanabe et al. of Sony Corp. disclose a photovoltaic-charged secondary battery with a combination of a photoelectric conversion device and a storage battery used as a dry battery. The photovoltaic-charged secondary battery is provided with a cylindrical core, a photoelectric conversion sheet wound to be freely drawn out from the cylindrical core, a storage battery capable of charging and discharging, and a control circuit. The storage battery is charged by the photoelectric conversion sheet wound on the cylindrical space, if necessary, and the charged storage battery is separated from a cylindrical assembly to be used as a dry battery.
However, the conventional photovoltaic-charged secondary battery has the following problems.
1) Since energy generated from a photovoltaic cell has a low driving voltage of −0.4 to 0.9V and a low energy density, the photovoltaic cell is formed with a large area. Accordingly, the photovoltaic-charged secondary battery is fabricated in the form of a module in which a plurality of photovoltaic cells is connected. Moreover, since it is necessary to artificially make a voltage suitable for the charging voltage of the secondary battery, i.e., 3.6 to 4.2V, the interfacial resistance is concentrated on the connection surface, and recombination of electric charges occurs during transfer, and thus the modulated photovoltaic cells show a very low efficiency compared with a unit cell [Kessler et al. Thin Solid Films, 480-481(2006)491].
2) The conventional technology of the integration of the photoelectric cell and the storage battery, disclosed in Japanese Publication Nos. JP2005-079031, JP2004-241228 and JP2005-209458 by Miyasaka et al., converts optical energy into electric energy using one electrode and stores the electric energy at the same time. However, since the electrode has the same potential, the charging energy is readily saturated, which results in a small capacity, and self-discharge of the electrode occurs readily. Moreover, the deterioration proceeds rapidly and thus the cycle lifespan does not exceed ten times.
3) The photovoltaic-charged secondary battery provided with the cylindrical core, the photoelectric conversion sheet wound to be freely drawn out from the cylindrical core, and the storage battery capable of charging and discharging, disclosed in Japanese Patent Publication Nos. JP2001-023701, JP2001-102094, JP2001-043903 and JP2001-076768, and Korean Patent Publication No. KP2001-0015248, has the same structure in which a plurality of photovoltaic cells is combined. Accordingly, the power generation efficiency and the charging performance are likely to deteriorate due to the low energy efficiency and the photoelectric conversion sheet which is rolled up and readily damaged.
4) It is required to provide a high efficiency battery capable of coping with the speed of electricity generation of a photoelectric conversion layer when charging the battery. Since a secondary battery placed at the bottom of the photoelectric conversion layer is heated to a high temperature by the received light, the internal pressure may excessively increase by the decomposition of electrolyte due to the high temperature. Accordingly, it is necessary to develop a suitable material and the associated system.
As described above, two kinds of elements including the optical power generation function and the charging function are mechanically combined or integrated into one electrode material according to the conventional technologies. However, there is not yet any method, in which optical energy is converted into electricity and, at the same time, immediately charged into a secondary battery to reduce energy loss, the potential difference between electrodes are systematically controlled to maximize the conversion efficiency of optical energy, maximize the energy utilization rate, and provide a long life span.
Accordingly, it is necessary to develop a photovoltaic-charged secondary battery system in which an electrode for converting optical energy into electricity and an electrode for charging the electrical energy have independent functions and are integrated to solve the above-described conventional problems.